1. Field of the Invention
The present invention relates to a power-assisted steering system for use in wheeled vehicles, and more particularly to a control apparatus for the power-assisted steering system of the type which includes an electrically operated control mechanism associated with a servo valve to vary a steering characteristic between input and output shafts of the servo valve in accordance with a control current applied thereto.
2. Description of the Prior Art
In such a conventional control apparatus as described above, the control current is applied to the electrically operated control mechanism to control a power assist to the driver's steering effort in accordance with a travel speed of the vehicle. Under such control, satisfactory results are obtainable in the turning characteristic of the steering wheel but unobtainable in the return characteristic of the steering wheel. Disclosed in Japanese Patent Early Publication No. 62-279170 is a control apparatus for a power-assisted steering system which comprises a servo valve arranged to be operated by relative rotation of input and output shafts for controlling the supply of fluid under pressure to a hydraulic power cylinder, a hydraulic reaction mechanism associated with the servo valve to apply a reaction force to the input shaft in accordance with fluid under pressure applied thereto so as to facilitate return movement of the steering wheel to its neutral position, a first sensor for detecting a steering angle of the steering wheel, a second sensor for detecting approach of the steering angle to the neutral position, and means for increasing the supply quantity of fluid under pressure to the reaction mechanism in response to an output signal of the second sensor. In this control apparatus, however, the supply quantity of fluid under pressure to the reaction mechanism is increased irrespectively of a travel speed of the vehicle. For this reason, if it is intended to facilitate return movement of the steering wheel during low speed travel of the vehicle, the steering wheel will be rapidly returned to its neutral position during high speed travel of the vehicle, resulting in deterioration of the driver's operational feel.